Which FI system is best & why? I just watched George's video (most of it) & now I'm curious.

So you're lookin' for a philosophical discussion, eh? Given that I'm not aware of anyone that has swapped one onto another engine (1.7 and 2L got D-, 1.8L got L-) that's pretty much what this is.

But one can look at subsequent history and see which general design was more widely accepted.

Speaking VEEEEERY generally, the primary difference between D- ("Druck" or pressure) and L- ("Luft" or air) is the primary input to the ECU to determine fuel flow. The D- system uses manifold pressure and TPS to determine load, where the L- uses a flapper to infer air mass.

So which one worked better? Well, lots of manufacturers used both. Volvo, Mercedes, Renault, VW, Porsche, Saab (and likely more) all used L-Jet. Alfa, Datsun, Honda, Peugot, Ford, Isuzu, Subaru, Toyota, ad nausea used L-Jet. In fact, I think the first-gen Miata used L-Jet or something damned close to it (I think my first-gen MR2 does, too).

But the primary downside with L-Jet is the flapper; it restricts airflow. And with improved sensors (that damned MPS), better TPS and temp sensors, and improved ECU computing power, the non-flapper D-Jet designs excelled; you'd be hard-pressed to find any EFI system, even aftermarket, that uses the flapper anymore.

So which general design was better? History says it was probably Druck-based system; that's what most modern cars use. I just replaced my D-Jet with a Microsquirt system that, with the exception of specific sensors and ECU, was pretty much the same concept.

Personally, I was a really big fan of the K-Jetronic "Continuous Injection System" in my Rabbit GTI (K- being "kontinuerlich" or continuous). As long as you kept the fuel tank clean it would run forever (but get rust in the tank and you're fooked).

And...discuss.

GA

Kept maintained, they both perform well and are reliable and inexpensive to maintain. Millions of cars, not just P-cars, used both versions. D-Jet was first, then came L-Jet and variations persist to this day on contemporary cars.

I can tell you that the wiring harnesses for D-Jet have fewer circuits which makes them less complicated and less costly to replace than L-Jet harnesses.

I have owned cars with both systems and would not hesitate to own either one.

Expert Testimony

The D-Jet was Bosch's first commercial fuel injection. L-Jet was Bosch's second commercial fuel injection. There were improvements incorporated into L-Jet to simplify it and make it more reliable.

I have put L-Jet on a 2.0 engine, and it ran great. So both fuel systems are very good, for the era they were built.

The L-Jet system evolved into the Motronic system that is currently used on all Porsche cars (except the Taycan).

Clay

with mr. b

germans basically invented electronic fuel injection.
..........and kept on reinventing it. good thing for everyone.

vw never hesitated pulling the trigger.
first D jet car (VW t3 fastback).
first L jet cars (VW 412, VWP 914 1.8).
someone's always got to lead the way.

astonishing that the systems still operate in cars after 50+ years.
testament to military grade engineering and german caution/care with new tech.

I always thought Bendix invented electronic fuel injection?

Word. Clipped rom Wikipedia:

The first commercial electronicially controlled manifold injection system was the Electrojector developed by Bendix and was offered by American Motors Corporation (AMC) in 1957.[34][35] Initial problems with the Electrojector meant only pre-production cars had it installed so very few cars were sold[36] and none were made available to the public.[37] The EFI system in the Rambler worked well in warm weather, but was difficult to start in cooler temperatures.[38]

Chrysler offered Electrojector on the 1958 Chrysler 300D, DeSoto Adventurer, Dodge D-500, and Plymouth Fury, arguably the first series-production cars equipped with an EFI system.[39] The Electrojector patents were subsequently sold to Bosch, who developed the Electrojector into the Bosch D-Jetronic. The D in D-Jetronic stands for Druckfühlergesteuert, German for "pressure-sensor controlled"). The D-Jetronic was first used on the VW 1600TL/E in 1967. This was a speed/density system, using engine speed and intake manifold air density to calculate "air mass" flow rate and thus fuel requirements.

Bosch superseded the D-Jetronic system with the K-Jetronic and L-Jetronic systems for 1974, though some cars (such as the Volvo 164) continued using D-Jetronic for the following several years. The L-Jetronic uses a mechanical airflow meter (L for Luft, German for "air") that produces a signal that is proportional to volume flow rate. This approach required additional sensors to measure the atmospheric pressure and temperature, to calculate mass flow rate. L-Jetronic was widely adopted on European cars of that period, and a few Japanese models a short time later.

Bendix put electronic fuel injection on a Chryslerin the 50s. But it used tubes, and you had to warm up the tubes before you could start it, and vibration would cause tube failure. They never put it into production. They sold it to Bosch who built it with the brand new transistor tech in the early 60s.

Clay

wow. amc. never knew that. they were an interesting company weren't they.

i'd have to bone up on it all again, but think i remember that america did all the heavy lifting work on the development of the transistor?

important to remember germany had a context for being enthusiast about fuel injection, admittedly high pressure style mechanical injection - from WW2 on. perhaps reason for bosch grabbing the bendix tech. its all a bit of an german/american partnership post WW2 too. space program etc. the best of both worlds.

wasn't the bosch proto D installed on a mercedes as a bosch skunk works project?
but merc hesitated so they drove down the road to VW who jumped straight in.

sent me down the rabbit hole.
1 of those chryslers left.
another league of hard to find parts in a galaxy beyond D jet!

https://www.allpar.com/threads/1958-chrysle...jection.228433/

Thanks for the link - great article about a special car. . .like the tag that reads, “ Bendix Aviation Corp.”

They both have strong points and weaknesses.

D-jet, as mentioned, doesn't place any extra restrictions on the incoming air. It also is more tolerant of small vacuum leaks.

On the downside, it is using a fairly imprecise method of working out how much air is going into the engine. It has to approximate it from using a measured pressure and the RPM. It has to make a lot of assumptions about the motor to use those to calculate the air that is going in.

L-jet directly measures the volume of the air that is going in with the air-flow meter. This gives it a much more precise idea of the actual amount of air going in. That can allow for much better mixture control, and it can cope better with changes to the engine than D-jet can.

The down-sides are that it places a restriction in the air flow path (the flap in the air flow meter), and that it is much less tolerant of even minor vacuum leaks than D-jet is.


Pick your poison. D-jet is associated with the more powerful 914 motors (2.0s and the high-compression 1.7s), but then again most modern fuel injection systems are based on the concepts used in L-jet, and many modern FI systems are direct descendants of it.

--DD